WLAN Building Blocks
At the most basic level, a wireless medium has no inherent organization. For example, a PC with wireless capability can simply bring up its wireless adapter anywhere at any time. Naturally, there must be something else that can also send and receive over the wireless media before the PC can communicate.
TIP In IEEE 802.11 terminology, any group of wireless devices is known as a service set. The devices must share a common service set identifier (SSID), which is a text string included in every frame sent. If the SSIDs match across the sender and receiver, the two devices can communicate.
The PC, as an end-user station, becomes a client of the wireless network. It must have a wireless network adapter and a supplicant, or software that interacts with the wireless protocols.
The 802.11 standards allow two or more wireless clients to communicate directly with each other, with no other means of network connectivity. This is known as an ad-hoc wireless network, or an Independent Basic Service Set (IBSS), as shown in part A of Figure 17-2.
Figure 17-2 A Comparison of Wireless Service Sets
Figure 17-2 A Comparison of Wireless Service Sets
No inherent control exists over the number of devices that can transmit and receive frames over a wireless medium. As well, many variables exist that can affect whether a wireless station can receive from or transmit to other stations. This makes providing reliable wireless access to all stations difficult.
An 802.11 Basic Service Set (BSS) centralizes access and control over a group of wireless devices by placing an access point (AP) as the hub of the service set. Any wireless client attempting to use the wireless network must first arrange a membership with the AP. The AP can require any of the following criteria before allowing a client to join:
■ A matching SSID
■ A compatible wireless data rate
■ Authentication credentials
Membership with the AP is called an association. The client must send an association request message, and the AP grants or denies the request by sending an association reply message. Once associated, all communications to and from the client must pass through the AP, as shown in part B of Figure 17-2. Clients can't communicate directly with each other, as in an ad-hoc network or IBSS.
TIP Keep in mind that regardless of the association status, any PC is capable of listening to or receiving the frames that are sent over a wireless medium. Frames are freely available over the air to anyone who is within range to receive them.
A wireless AP isn't a passive device like an Ethernet hub, however. An AP manages its wireless network, advertises its own existence so that clients can associate, and controls the communication process. For example, recall that every data frame sent successfully (without a collision) over a wireless medium must be acknowledged. The AP is responsible for sending the acknowledgement frames back to the sending stations.
Notice that a BSS involves a single AP and no explicit connection into a regular Ethernet network. In that setting, the AP and its associated clients make up a standalone network.
An AP can also uplink into an Ethernet network because it has both wireless and wired capabilities. If APs are placed at different geographic locations, they can all be interconnected by a switched infrastructure. This is called an 802.11 Extended Service Set (ESS), as shown in part C of Figure 17-2.
In an ESS, a wireless client can associate with one AP while it is physically located near that AP. If the client later moves to a different location, it can associate with a different nearby AP. The
802.11 standards also define a method to allow the client to roam or to be passed from one AP to another as its location changes.
Continue reading here: Access Point Operation
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